Impact of Soil Modeling on Fatigue Design of Lazy Wave Riser Systems

The Liza risers comprise production risers, water injection risers and gas injection risers, and a lazy wave configuration is selected considering FPSO motion, reservoir fluid and overall project execution requirements. During operation, the risers are expected to move cyclically with small vertical displacement amplitudes (e.g. 0.1% to 1% of the riser diameter), and a key design issue is the fatigue life of these risers at critical locations including the touch-down zone which will be governed by the seabed stiffness. The role of soil response on fatigue life of riser with buoyancy has been investigated through nonlinear finite element and comprehensive lab and field testing program. Published methodologies for determining seabed stiffness values for risers concentrate more on larger amplitude motions based on the design requirements of steel catenary risers. The paper presents the sensitivity of the fatigue life at TDP to various soil model and provides insight in the results. Also included is the importance of site specific soil investigation in the context of design of riser.

Fatigue Damage Study of Helical Wires in Catenary Unbonded Flexible Riser Near Touchdown Point

This study presents an analytical model of flexible riser and implements it into finite-element software abaqus to investigate the fatigue damage of helical wires near touchdown point (TDP). In the analytical model, the interlayer contact pressure is simulated by setting up springs between adjacent interlayers. The spring stiffness is iteratively updated based on the interlayer penetration and separation conditions in the axisymmetric analysis. During the bending behavior, the axial stress of helical wire along the circumferential direction is traced to determine whether the axial force overcomes the interlayer friction force and thus lead to sliding. Based on the experimental data in the literature, the model is verified. The present study implements this model into abaqus to carry out the global analysis of the catenary flexible riser. In the global analysis, the riser–seabed interaction is simulated by using a hysteretic seabed model in the literature. The effect of the seabed stiffness and interlayer friction on the fatigue damage of helical wire near touchdown point is parametrically studied, and the results indicate that these two aspects significantly affect the helical wire fatigue damage, and the sliding of helical wires should be taken into account in the global analysis for accurate prediction of fatigue damage. Meanwhile, different from the steel catenary riser, high seabed stiffness may not correspond to high fatigue damage of helical wires.

Numerical Model for Pipeline Laying During S-lay

The S-lay method has been widely used in pipeline installation from shallow water to deep water for decades. In this paper, a novel numerical model for analyzing pipelines in the S-lay problem is proposed to investigate the overall configuration, internal forces, and strain of the pipeline taking into account the influence of ocean currents and seabed stiffness. The influence of many important factors, including the variation position of the liftoff point, the change of stinger radius, ocean currents, seabed stiffness are investigated in detail. Some useful results are obtained: the stress state of the pipeline is found to vary greatly during the whole laying process; the train of the pipeline at both the upper and the lower sides is very important; ocean currents have negligible influence on the pipeline; traditional “touchdown factor” is not suitable to predict the real pipe embedment; and soil stiffness plays an important role in pipeline behavior on the seabed. The illustrative examples and comparison with a previous work demonstrate the widespread applicability of this model. Moreover, the solution process of this model is easy and fast, so it is suitable for engineering applications.

Numerical Analysis for Pipeline Installation by S-Lay Method

S-lay method is widely used in pipeline installation from shallow water to deep water for decades. However, previous researches on S-lay method are not so complete because they are based on some assumptions, such as calm water and rigid seabed, which make the results deviate from precision. In this paper, a novel numerical model for analyzing pipelines in the S-lay problem is proposed to investigate the overall configuration, internal forces and strain of the pipeline taking into account the influence of ocean currents and seabed stiffness. Such a model is of great advantage because it can properly consider many influential factors in the real situation, including the variation position of the liftoff point, the change of stinger radius, the influence of ocean currents, seabed stiffness and the strain of the pipeline. Moreover, the solution process of this model is easy and not time consuming, so it is suitable for engineering application.

Centrifuge Modelling of Riser-Soil Stiffness Degradation in the Touchdown Zone of a Steel Catenary Riser

Steel catenary risers (SCRs) are economical to assemble and install compared to conventional vertical risers. However, accurate evaluation of the fatigue life of an SCR remains a major challenge due to uncertainty surrounding the interaction forces at the seabed within the touchdown zone (TDZ). Fatigue life predictions are heavily dependant on the assumed stiffness between the riser and the seabed and therefore an accurate assessment of seabed stiffness — or more specifically the nonlinear pipe-soil resistance — is required. During the lifespan of an SCR, vessel motions due to environmental loading cause repeated penetration of the riser into the seabed within the TDZ. This behaviour makes assessment of seabed stiffness difficult due to the gross deformations of the seabed and the resulting soil remoulding and water entrainment. This paper describes a model test in which the movement of a length of riser pipe was simulated within the geotechnical beam centrifuge at the University of Western Australia. The model soil was soft, lightly over-consolidated kaolin clay with a linearly increasing shear strength profile with depth, typical of deepwater conditions. The pipe was cycled over a fixed vertical distance from an invert embedment of 0.5 diameters to above the soil surface. This range represents a typical vertical oscillation range of a section of riser within the TDZ during storm loading. The results indicate a significant degradation in the vertical pipe-soil resistance during cyclic vertical movements. Due to the cyclic degradation in soil strength, the component of the vertical resistance created by buoyancy was significant, particularly due to the influence of heave. A new approach to the interpretation of heave-enhanced buoyancy was used to extract the separate influences of soil strength and buoyancy, allowing the cyclic degradation in strength to be quantified. During cycling, the soil strength reduced by a factor of 7.5 relative to the initial penetration stage. This degradation was more significant than the reduction in soil strength during a cyclic T-bar penetration test. This contrast can be attributed to the breakaway of the pipe from the soil surface which allowed water entrainment. This dramatic loss of strength and therefore secant stiffness, and the significance of the buoyancy term in the total vertical pipe-soil resistance, has implications for the fatigue assessment of SCRs.